POOR weather over summer has resulted in a lucrative black market for the sale of mud crabs in Queensland.

Recreational fishermen have been caught taking to websites such as Facebook, eBay and Gumtree to illegally sell mud crabs for up to $50 to try to reap the benefits of a poor crabbing season.

It comes as the Queensland Boating and Fisheries Patrol was preparing for a busy long weekend as thousands of people were taking to the water over the Easter break.

Fishers and crabbers have also been found resorting to illegal activities such as keeping undersized and female mud crabs after a dry summer saw fewer mud crabs being caught in Queensland's waterways.

Earlier this year, a man was fined $3100 after pleading guilty to five crab-related offences, including possessing undersized and female mud crabs. During his sentencing at the Brisbane Magistrates Court, the magistrate referred to the man's "greed and disregard for the future sustainability of crab stocks".

Queensland Boating and Fisheries Patrol field officer Vaughan Heath said operations against unlawful crabbing had been conducted in the Kedron Brook floodway in Brisbane.

Online groups have taken things into their own hands by "naming and shaming" culprits found illegally selling seafood without commercial licences on sites such as Facebook.

One such group, Crab Watch Queensland - operated by a man who wanted to be known as Crab Man - said he hoped social media would help authorities stamp out the illegal sale of crabs.

"There's been quite a few people selling crabs online this year, and we have reported them to Fisheries (Queensland)," he said.

"Some people don't realise they're doing something wrong, but without a commercial licence, you can't sell these crabs online."

Most recent figures obtained by The Sunday Mail reveal a decline in the number of licences being issued to people who are able to access the commercial crab fishery.

There are now 412 licences in Queensland, according to the Department of Fisheries and Agriculture, but the actual number of operators is "somewhat less than this number" as people may hold multiple fishing licences. Five years ago, there were 437 licences.

A Department of Fisheries and Agriculture spokesperson said strict rules applied to the commercial take of crabs.